The present invention relates to graphics plotters and, more particularly, in a graphics plotter system having a platen over which paper is moved longitudinally to create one axis, penholding apparatus moved transversely across the platen and the paper to create the other axis, and a controlled raising and lowering mechanism connected to the penholding apparatus for lowering a pen being held therein into contact with and for raising it from contact with the paper and for providing a relative vertical position value signal, to the improvement for allowing known pen spacing from the paper during pen movement comprising, vertical sensing means carried by the penholding apparatus for sensing the top surface of the platen; means operably connected to the vertical sensing means and to receive the vertical position value signal from the controlled raising and lowering mechanism for calculating and saving a plurality of deviation values indicating the distance deviation of the platen top surface from parallel at a plurality of spaced points across the platen; and, means connected to the controlled raising and lowering mechanism for adjusting the position of the penholding apparatus as a function of the deviation values to position the penholding apparatus at known distances from the top surface of the platen as the penholding apparatus is moved transversely across the platen.
The original graphics plotters as employed with computers to create two-dimensional graphics and drawings comprised a flat table upon which the drawing paper was fastened and over which a printhead assembly was moved by a beam also moving on spaced tracks parallel to one another. Such plotters were large and cumbersome, particularly with larger so-called "D" and "E" sized drawings as typically used in commercial applications. More recently, a single, non-moving-beam system such as that represented by the critical elements thereof in FIG. 1 has been employed as the standard approach to plotting. In such a plotter, there is a platen 10 over which the paper (not shown) is draped to be moved longitudinally for one axis by a pair of driven pinch-rollers 12 positioned at the respective edges of the paper. Ideally, the top surface of the platen 10 is completely straight. The single beam 14 (typically in the form of a rod or bar) is disposed above and parallel to the top surface of the platen 10. Again, ideally, the beam 14 is completely straight and parallel to the top surface of the platen 10. A driven penholder 16 releasably gripping a pen 18 is moved transversely across the paper from side to side to create the other axis. An appropriate mechanism (not shown) is connected to raise and lower the pen 18 as by rotating the penholder 16 about the beam 14. As is well known, the pen 18 is lifted during movement from point to point on the drawing wherein no line is to be created and then lowered to contact the paper for subsequent movement creating the lines of the drawing. Ideally, as indicated in FIG. 1, the writing tip 20 of the pen 18 moves transversely over the top surface of the platen 10 at a constant spacing when raised. Unfortunately, it is impossible to mechanically create the ideal situation of FIG. 1. In actuality, there is component misalignment and mis-shaping in the manner of FIG. 2. Either or both the platen 10 and beam 14 can be skewed vertically and/or horizontally from the desired true horizontal and parallel positions. Thus, even if straight, the platen 10 and the beam 14 could diverge and converge creating differences in the spacing therebetween. Additionally, both the platen 10 and beam 14 may be flexed or otherwise distorted along the length thereof from a true straight condition. This, too, creates differences in the spacing between the writing tip 20 and the top of the platen 10 as the pen 18 is moved from side to side over the platen 10.
It should be noted at this point that the deviations from true straight and parallel are typically quite small. Unfortunately, the desired operating clearances are also quite small for purposes to be addressed shortly hereinafter. The result is as shown in FIGS. 3 and 4, which are exaggerated for purposes of demonstration as was FIG. 2. Assuming a simple upward flexing of the platen 10, if the clearance distance between the writing tip 20 and the top of the platen 10 is set to its desired minimal clearance at the outer edges, the writing tip 20 may actually strike the platen 10 upon reaching the raised center portion thereof and, in an extreme situation, could be bent or broken off as depicted in FIG. 3. At the least, it could contact the paper and make undesired marks thereon. As depicted in FIG. 4, if the clearance is set at the higher center portion, there will be far too much clearance at the lower outer edges. This of course begs the question as to why a minimal clearance is desirable or necessary. There are several reasons which can be best understood with reference to FIGS. 5 and 6. As depicted in FIG. 5, the pen 18 is raised and lowered (referred to in the art as "dropped") by rotating the driven penholder 16 about the cylindrical beam 14 as indicated by the arrows 22. Again in greatly exaggerated representation, FIG. 6 shows the result of pen dropping by rotation. As can be seen and appreciated from the drawing, since the writing tip 20 is moving along a circular arc (as opposed to being raised and lowered along a vertical line), if the arc of movement is large (relatively speaking), there is an appreciable deviation between the vertical position of the writing tip 20 above the paper and its actual point of contact with the paper upon being dropped. Thus, one aspect of knowing the drop distance is the ability to minimize the difference between desired pen position and actual pen position resulting from the rotational dropping of the pen; that is, by knowing the drop distance at any point, adjustment for the delta P shown in FIG. 6 can be affected by the logic so that the actual pen position on the paper is identical to the desired pen position.
Another aspect of a minimal drop distance is the maximization of through-put. Typically, the pen 18 is moved in a raised position to a desired point of line beginning. Movement then stops while the pen is dropped and doesn't continue until the dropping action has been completed. Again, while the differences in time involved between a minimal drop distance and large drop distance are in fractions of a second, in modern highspeed plotters, this time when accumulated over a continuing series of pen raising and lowering (as when drawing short-vectored alphanumeric text materials) can significantly affect the through-put of the plotter. The ability to attain a known drop distance also potentially affects the through-put of plotters in general through the changing of the manner in which they operate. At present, as mentioned above, movement of the pen 18 along the two axes is stopped while the pen 18 is raised and lowered. With the ability to attain a known drop distance, it would be possible to eliminate the stopping and starting during pen raising and lowering. By being able to accurately anticipate the point at which to begin pen lowering in order to have the writing tip 20 contact the paper at the desired point of line beginning, through-put would be increased even further.
Wherefore, it is an object of the present invention to provide a method and apparatus for mapping the platen in a graphics plotter to provide run-time compensation for deviations between a true straight and parallel orientation between the writing tip of the pen and the top surface of the platen.
It is another object of the present invention to provide a method and apparatus for adjusting the pen down position for accuracy in a graphics plotter in which the pen is dropped by rotating it about a horizontal axis.
It is still another object of the present invention to provide a method and apparatus for maximizing throughput as a function of the pen down time consumption in a graphics plotter in which the pen is dropped by rotating it about a horizontal axis.
Other objects and benefits of the present invention will become apparent from the detailed description contained hereinafter taken in conjunction with the accompanying drawing figures.